The advantages of displaying an interlaced video signal in non-interlaced or "progressive scan" form are well known and a number of arrangements have been proposed for providing such a scan conversion. For example, in U.S. Pat. No. 4,415,931 entitled TELEVISION DISPLAY WITH DOUBLED HORIZONTAL LINES which issued Nov. 15, 1983 to R. A. Dischert, progressive scan conversion of a luminance signal is obtained by displaying each incoming horizontal line twice thereby doubling the number of displayed raster lines. This is achieved by storing each incoming interlaced line in one of two memories. As one of the memories is being "written", the other memory is read twice thereby providing two lines of time compressed non-interlaced video for each line of interlaced video processed.
As another example, K. H. Powers describes a progressive scan converter in U.S. Pat. No. 4,400,719 entitled TELEVISION DISPLAY SYSTEM WITH REDUCED LINE-SCAN ARTIFACTS which issued Aug. 23, 1983, wherein the additional scan lines added to the video output signal are obtained by means of interpolation of the incoming scan lines.
It has been recognized by Pritchard et al. in U.S. Pat. application Ser. No. 526,702 filed Aug. 26, 1983 (now U.S. Pat. No. 4,558,347 which issued Dec. 10, 1985), entitled PROGRESSIVE SCAN TELEVSION SYSTEM EMPLOYING VERTICAL DETAIL ENHANCEMENT (assigned to the same assignee as the present application, and incorporated herein by reference) that interpolation, being essentially an averaging process, tends to reduce the vertical detail content of the interpolated signal. In that system, a line comb filter is used to provide luma-chroma separation and interpolation of the luma signal prior to time compression (video speed-up) in the converter speed-up memories. To restore the vertical detail lost by comb filtering, a vertical detail enhancement signal is generated by low pass filtering the chroma output of the comb filter. The low frequency vertical detail enhancement signal thus obtained is then combined with the "real" (i.e., received) and the interpolated lines to restore the vertical detail lost by comb filtering. In accordance with one aspect of their invention, the polarity of the vertical detail enhancement signal is alternated from line-to-line to effect preshoot and overshoot enhancement of the scan converted video output signal. Pritchard et al. further suggest that the vertical detail signal be subjected to a non-linear transfer function (e.g., coring, peaking, paring as described in U.S. Pat. No. 4,245,237 of Lagoni) for the purpose of providing an optimum balance between subjective enhancement and flicker of displayed images.
In the aforementioned scan conversion systems, the video input signal is of composite form and conversion to color component (RGB) form for subsequent kinescope display is performed subsequent to speed-up of the video signal. A scan converter suitable for use with video sources of either composite form or RGB form is described in the commonly assigned U.S. Pat. Application of Dorsey et al. entitled VIDEO SIGNAL PROCESSOR FOR PROGRESSIVE SCANNING, Ser. No. 591,996 filed Mar. 21, 1984 which issued Feb. 25, 1986, as U.S. Pat. No. 4,573,068 and incorporated by reference herein. In the Dorsey et al. system, the composite video input signal is comb filtered, detail enhanced and converted to RGB form prior to being time compressed to double the line rate. The RGB video input signals are coupled directly to the time compression (speed-up) memories without further processing. As noted by Dorsey et al., the RGB video input signal may be a computer that internally generates RGB signals with high horizontal and vertical resolution and so the vertical detail enhancement used for processing of the composite video signal to overcome the effects of comb filtering is not needed when processing the RGB format video input signal.